The bright light emitted by an iPad could give some people reading before bedtime a bout of insomnia, researchers suggest.

This is because the iPad uses a back-lit display rather than the "e-paper" found in other popular e-readers such as the Kindle that mimic the printed, duller page by reflecting light from elsewhere.

"If you're using a Kindle — which doesn’t use a significant light source — that may potentially have less of an impact compared to a device like a laptop or an iPad with more significant light exposure," said Alon Avidan, a neurologist and associate director of the Sleep Disorders Center at the University of California, Los Angeles (UCLA).

Sleep researchers have long known that light inhibits the release of melatonin, a hormone that settles the body into sleep mode toward evening time. "Melatonin is pro-sleep," said Frisca Yan-Go, director of the UCLA Sleep Disorders Center.

Any artificial light source at night, whether it be from a television or a bedside lamp, can mess with the body's melatonin production, Yan-Go said, which rises when darkness falls, then tapers off toward morning as part of a natural cycle called the Circadian rhythm.

According to Yan-Go and other sleep researchers, the reason for concern with the iPad over other forms of late-night light is that readers hold the device relatively close to their face. In theory, this could mean that the iPad affects the body's melatonin cycle more than watching late-night reruns on a television that is clear across the room, Yan-Go said.

The fact that the iPad has a full-color screen — blue light in particular sparks alertness — and offers Web browsing and other engaging computer activity, unlike other dedicated e-readers, could also keep people from getting a good night's rest, scientists say.

Yan-go said that she has not yet had patients complain about iPad-induced insomnia — perhaps because the device is so new – but she nevertheless recommends against using it as an e-reader given past cases of patient restlessness related to laptops and television. "You really want the [sleep] environment to be quiet and dark," Yan-go said.

E-reading before bed
The centuries-old tradition of reading a paperbound book before retiring has been going digital recently. Sales of e-readers continue to boom and are expected to nearly triple from about four million sold last year to over 11 million in 2010, according to a new report by Digitimes Research.

Massachusetts-based E Ink Corp, which makes the e-paper used by major e-readers including Amazon's Kindle and Barnes & Noble's Nook, aims to provide a "true book-like experience" for its customers, according to the company's Web site.

E-paper technology simulates the look of ink on paper on a screen by electrically arranging little charged capsules filled with black- or white-pigmented particles.

Rather than emitting light of its own, like traditional computer screens and the iPad, an e-paper screen reflects ambient light in a room or outdoors, just as a real book would. This makes e-paper devices easy to view even in direct sunlight, a big plus for consumers with the summertime reading season right around the corner.

The iPad, meanwhile, uses a 262,000-color, liquid crystal display (LCD) that must be back-lit because these liquid crystals do not shine on their own. Using an iPad to read at night does avoid the complication of waking a partner by turning on the bedside lamp or having to use a flashlight under the covers.

Yet because the iPad beams light directly into a user's eyes, reading on it is similar to working on a computer late at night — another practice that sleep experts advise against when it comes to preparing for imminent slumber.

Seeing blues while racking up Web views
Beyond light emission, the iPad's colorful display could also contribute to insomnia when compared to the black-and-white screen of a Kindle, said Avidan.

Cells in our eyes are more sensitive to blue light than other colors, a characteristic that has evolved in humans to triggers wakefulness upon the sight of an azure sky in the morning. "When you look at an LCD screen that has more rich colors in the blue range, that could conceivably have an additive effect" on insomnia, Avidan said.

The temptation to do work on an iPad could also keep people from snoozing. "The problem with someone surfing the Web with an iPad is that it's engaging," Avidan said, whereas "the Kindle is more for someone who is just reading."

Lights out
Overall, Avidan thinks that the risk to most people losing sleep from e-readers is minimal. "For a typical person who doesn’t have problems falling asleep, if they're using a Kindle or an iPad or whatever, they probably don’t need to worry," Avidan told TechNewsDaily. "But if they suffer from insomnia and have difficulty falling asleep ... then using artificial light from, say an iPad or a laptop, that certainly can have a detrimental effect."

Other researchers are skeptical, however, that the iPad and other electronic devices pose all that much of a hindrance to getting some shut-eye.

"The amount of light coming off a computer or an iPad is not anywhere near bright enough to set Circadian cycles," said Michael Marmor, a professor of ophthalmology at Stanford University.

"Outdoor light is many thousand-fold brighter than any indoor computer or ordinary room light," he said, which is why jet-lagged travelers are told to get outside to help reset and gauge their internal clocks to the local day-night norms.

"I think people should not worry about lighting effects from either the Kindle or the iPad," said Marmor. "What will keep you awake is the news of the day ... or a really good book."